Absorption spectroscopy, including tunable diode laser absorption spectroscopy (TDLAS) are techniques for measuring the concentration of various species in a gaseous mixture. Absorption spectroscopy techniques are particularly well-suited to achieve very low detection limits. In addition to species concentration, it is possible with absorption spectroscopy to determine the temperature, pressure, velocity and mass flux of certain species of the gas under observation.
A typical TDLAS apparatus includes a tunable diode laser light source plus emission and receiving optics and detectors. The output of the tunable diode laser is tuned over a wavelength range encompassing selected absorption lines of various gas species of interest in the path of the laser beam. Absorption features cause a reduction of measured signal intensity which can be detected and used to determine gas concentration and other properties. The use of TDLAS is described in detail in co-pending U.S. patent application Ser. No. 10/543,288 entitled “Method And Apparatus For The Monitoring And Control Of Combustion”, which application is incorporated herein by reference in its entirety.
Tomography is a technique whereby spatial resolution is obtained from line-of-sight measurements over multiple, often intersecting paths or projections, at a variety of selected orientations. Tomography is a well-known technique which has been used extensively in applications such as medical imaging. At each orientation, the transmitted radiation is monitored. Each transmission measurement is an average over the path traversed by the beam. In other words, an individual projection provides no spatial information. Using the transmission results from many projections as inputs allows the use of mathematical transforms to reconstruct what the object must look like in order to produce the measured transmissions. In this way, spatial resolution is obtained from a technique that intrinsically produces a line-of-sight average. High tomographic spatial resolution requires that many projections be used. In the case of absorption spectroscopy applications the required optical access may not be available to support many beam paths thus limiting traditional tomography to relatively low resolution. In other cases, cost considerations may limit the number of possible beam paths also limiting the obtainable resolution.
Temperature binning is a second known absorption spectroscopy technique where a certain degree of spatial resolution can be achieved for temperature measurements taken along a single line-of-sight projection. See, for example, “Measurement of Nonuniform Temperature Distributions Using Line-of-Sight Absorption Spectroscopy” by Liu, Jeffries & Hanson (February 2007, AIAA Journal, 45:2:411) (Appendix II), which article is incorporated by reference herein in its entirety. Temperature binning techniques may supplement conventional TDLAS measurements to determine variations in temperature over a single beam path. For example, with conventional TDLAS a path averaged temperature may be determined by measuring absorption over two carefully-chosen wavelengths of the same combustion species. Sensitive temperature measurements can be made if the transmission intensity of the two chosen spectral features are known to behave differently as the temperature changes. The ratio of the intensity of the absorption features is particularly well suited as an indicator of temperature, since the ratio does not depend upon species concentration as does the measured intensity of each individual line. Thus accurate temperature measurements may be made with TDLAS based apparatus even though actual species concentration is initially unknown.
Temperature binning is similar to conventional two line temperature measurement, but the use of multiple absorption lines permits the determination of the relative length of more than one temperature zone along the line-of-sight path; the zones are known as temperature bins. For most combustion systems, water provides a convenient target species since it is ubiquitous in combustion systems and it absorbs strongly at readily accessible wavelengths. However, oxygen or any other species can be used as the target molecule in alternative gas systems as well. In most combustion systems, the temperature varies greatly over the path of any single projection. In summary, temperature binning involves the use of multiple wavelengths to identify bins of length L1 . . . Ln, where each defined bin is at an average temperature of T1 . . . Tn. In general, every additional bin requires that at least two new wavelengths be used to make the measurement. In cases where the species concentration of the target is not known, more than two wavelengths must be added to define every new bin.
Many temperature bins can be defined as long as a sufficiently large number of appropriate wavelengths are used for the measurement; however, the temperature binning technique has one notable shortcoming; binning does not provide information regarding how each bin is arranged spatially with respect to the other bins. Sometimes this information can be gained from a priori knowledge of the gas or combustion system. For example, relatively cool transition zones are often located at the edges of a combustion system. A priori knowledge may however lead to erroneous conclusions in certain instances, for example in the case of malfunctioning combustors or poorly understood gas systems.
The present invention is directed toward overcoming one or more of the problems discussed above.